Continuous casting of metals



June 20, 1967 L. w. COLLINS ETAL 3,326,270

CONTINUOUS CASTING OF METALS Filed June 12, 1964 gar g: Q

l/Vl/E/V TOR S United States Patent 3,326,270 CONTINUOUS CASTING 0F METALS Donald L. W. Collins, Moreton Pinkney, near Rugby, and Peter E. Sevier, Broughton, near Banbury, England, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed June 12, 1964, Ser. No. 374.635 Claims priority, application Great Britain, June 12, 1963, 23,468/ 63 4 Claims. (Cl. 164-89) The present invention relates to a process and apparatus for the continuous casting of metal.

The casting of large aluminium ingots for rolling and extrusion is usually carried out by the direct chill casting process, in which the metal is poured into an open-bottomed mould, having side walls defining an aperture having the same cross sectional shape as is desired for the ingot. The solidified lower portion of the ingot is supported on a movable stool positioned below the mould, the stool initially closing the bottom of the mould and being lowered at a rate determined by the desired rate of castmg.

In the now conventional direct chill continuous casting process for the production of aluminium ingots the mould is chilled so that contact between the molten metal and the mould wall produces a shell of solidified metal extending downwards from just below the meniscus formed between the molten metal and the mould. The solidified shell contracts and thus below the meniscus the outer surface of the ingot is not in contact with the chilled mould wall. Although some of the heat content of the molten metal poured into the mould is absorbed into the chilled mould structure, the greater part of the heat content of the metal is dissipated by coolant sprayed directly onto the surface of the ingot at a position just below the bottom margin of the mould. The chilling effect of the coolant applied directly to the surface of the ingot below the mould, in practice, extends upwards for a distance of about l-l /z inches, when the ingot is cast at a conventional casting rate.

It is a well-known disadvantage of the direct chill continuous casting process that the ingots produced by the process are liable to severe surface irregularities. These are mainly due to the formation of cold shuts which give the ingot a severely corrugated surface, or to bleeding, caused by molten metal which breaks through the solidified shell, runs down and solidifies on the external surface of the ingot. Cold shuts are formed as a result of any significant variation in the vertical level of the meniscus between the molten metal and the chilled mould wall. Bleeding occurs when the molten core of the ingot breaks through the thin solidified shell which has contracted away from the mould wall. The metal which breaks out through the shell solidifies on the external surface of the ingot at a level dependent on the extent of the chilling effect of the mould and that produced by the direct application of coolant to the surface of the ingot/Cold shuts may be obviated by increasing the casting rate, but this can lead to increased bleeding of the ingot.

In the case of aluminium ingots produced by conventional direct chill continuous casting it is necessary to scalp the ingots to produce a smooth surface before they can be rolled. Apart from the direct cost of the scalping operation, the size of the ingot is reduced by scalping, and it will readily be appreciated that the elimination of the scalping operation (or a substantial reduction of it) would lead to very substantial economies.

The best grain structure and freedom from segregation is produced in ingots of circular cross section if the pool of molten metal in the top end is, during casting,

3,326,270 Patented June 20, 1967 ICC approximately conical in shape with the apex at the lowest point of the pool. In the case of ingots of rectangular cross section the pool of molten metal should have the form of an inverted, triangular prism. The pool of molten metal does not, however, have this shape in an ingot produced by the use of conventional direct chill continuous casting techniques, wherein substantially no cooling of the surface of the ingot takes place between the level, where it first contacts and shrinks away from the cooled mould wall, and the level where the effect of the coolant applied directly to the surface of the ingot is felt.

It is found that the cold shuts and bleeding can be obviated or at least substantially reduced if the solidification of the surface to produce the shell takes place at a level so low down in the mould that the effect of the coolant applied below the mould will ensure the solidified metal does not remelt locally at the surface of the ingot. Further, by casting in such a manner that solidification first occurs within the zone in which the effect of the coolant applied to the ingot surface below the mould is apparent, the desired profile of the solidification front throughout the ingot may be achieved, thereby giving optimum grain structure and freedom from segregation. However, it is difficult to control the level of metal in a mould accurately and it is particularly difficult to maintain it at a substantially constant level only about 1 /2 inches from the bottom margin of the mould.

The method of the present invention accordingly depends upon the use of a mould, part of which is lined with heat insulating material to prevent the surface solidification of metal in an open-ended continuous casting mould above the desired level. Various forms of rigid heat insulating material have already been proposed for this purpose, but none of these has proved wholly satisfactory in use. Such rigid heat insulating material has required the cutting of an outwardly flaring face at the lower margin or the formation of a similar face on the surface of the permanent mould just below the lower margin of the heat insulating material.

The mechanical strength of rigid heat insulating materials, which will withstand attack by molten metal, is relatively poor and it is difficult to form such material so that its surface conforms to the wall of the mould so as to obtain support from it.

We have found surprisingly that it is unnecessary to utilise an outwardly flaring surface at the lower margin of the insulating material.

According to the present invention there is provided a method of continuously casting metal, principally aluminium, by the direct chill continuous casting process, characterised in that the mould wall above the level at which solidification takes place is lined with insulating material in thin flexible sheet form, said material being substantially unafiected by the molten metal at the casting temperature. In this way a pool of completely molten metal without a solidified periphery may be maintained in the mould above the level at which surface solidification is produced as the result of the direct application of coolant to the surface of the ingot below the mould and, moreover, a solidification front of the desired shape is produced in the top end of the ingot which is being poured.

A suitable flexible sheet material for use as insulating material in a mould is sold under the trademark Fiberfrax. This material is believed to consist of fibres of aluminium silicate made up into a material similar to paper and is substantially unaffected by contact with molten aluminium. The material may be obtained in the form of soft, flexible sheets of a thickness as little as 0.02 inch. Although such paperlike insulating material has little tensile strength, the drag effect of the descending molten aluminium at the speed used in continuous casting (34 inches/minute, for example) does not damage it excessively, although it it preferable to apply a coating of a silicone release agent to its surface before casting is commenced.

The head of molten metal in the thermally insulated portion of the mould presses the flexible sheet insulating material out against the mould wall which acts as a backing and support for the heat insulating material.

The flexible insulating mould lining is preferably secured in position in the mould by means of a retaining device which engages the top edge of the insulating sheet material and is positioned near the mouth of the mould above the level of the pool of molten metal, which is to be maintained in the mould during the continuous casting of an ingot.

The retaining device is shaped so as to hold the top edge of the sheet material against the mould wall, whilst the lower edge of the sheet material hangs free in close proximity to the mould wall before the pouring of an ingot is commenced. The pressure of the molten metal presses the lower portion of insulating sheet material outwardly into engagement with the wall of the mould during pouring, as stated above.

Although other methods of securing the insulating material in the mould are possible, for example, by means of adhesive, the use of clips or clamping bars is preferred, since this permits a change of lining material to be carried out very easily at suitable intervals.

The flexible sheet insulating material must have sufficient thermal insulating effect to prevent solidification at a level above the bottom edge of the material, since this will lead to drag marks being formed on the surface of the ingot and the early disintegration of the lining. In practical experiments carried out, using Fiberfrax flexible sheet insulating material, it was found that a thickness of insulating material of 0.040.08 inch (1-2 mms.) was suflicient and the thickness of this material could be decreased towards the bottom edge of the insulation, this being readily arranged by using two layers of'sheet material of unequal overall depth, the inner sheet preferably overlapping the outer sheet by about Mi-Vz inch. For in gots of circular cross section the bottom edge of the inner and longer layer may be split longitudinally at intervals to ensure good conformity of the sheet material with the mould wall under the pressure of the molten metal.

The bottom edge of the insulating layer is arranged at a distance from the bottom edge of the mould such that, under the casting conditions, the solidification front at the surface of the ingot does not rise up sufficiently far in the mould for dragging of solidified metal on the insulating sheet material to occur.

In work carried out with aluminium alloys the bottom edge of the Fiberfrax flexible sheet insulating material was found to be best positioned at a distance of about As-1 A inches (about 2232 mms.), preferably about 1 inch (about 25 mms.), from the bottom edge of the mould under the casting conditions employed. Using a mould partially insulated in this way it has been found possible to cast ingots with particularly smooth surfaces and having a very uniform metallurgical structure.

One arrangement of mould utilised for putting the pres ent invention into effect is illustrated in the accompanying drawings, wherein:

FIGURE 1 is a section through a mould.

FIGURE 2. is a plan view of the mould of FIGURE 1,

and

FIGURE 3 is a perspective view of a mould clamp.

The mould illustrated in FIGURE 1 is a conventional mould for continuous casting aluminium ingots for rolling into sheet. The mould 1 may conventionally be 3-4 inches (75-100 mms.) in depth and be provided with a mould box (not shown) for directing coolant water into the rear surface 2 of the mould and along the inclined surface 3, 'so that it impinges directly against the surface l of the ingot issuing from the mould to provide the principal cooling effect which the ingot is subjected to.

It is an extremely simple matter to apply the method of the present invention in conjunction with a conventional mould. Two layers, 4 and 5, of Fiberfrax thermal insulation paper are secured in position by means of a mould clamp 6, which comprises a bar 7, shaped to one side of the mould aperture, to which are secured a number of clamp arms 8, each of which is provided with a clamp screw 10, to engage the outer edge of the top flange of the mould 1. It will be understood that where a round mould is utilised to cast a conventional round extrusion billet, two or more part-circular bars 7, having radially extending clamp arms 8, would be employed. The insulation layers 4 and 5 are adhesively secured to the bar 7 by, for example, an adhesive based on sodium silicate before being placed in the mould. The insulating layers can thus be lifted into and out of the mould very simply and there is no difficulty in replacing the insulation layers quickly, whereas the replacement of rigid insulation inserts in a mould must take much longer and thus the use of flexible insulation material produces a considerable advantage, particularly where a number of moulds are held in a common casting table, as is conventional.

A further advantage lies in the fact that conventional casting lubricant may be applied to the metal mould before the insulation sheet material is put into position. It is found when using Fiberfrax material, no lubricant is required on the insulation material itself, but it is possible that with other flexible insulation materials, lubricant might be required.

In one example of the process of the invention casting was carried out using the mould shown in the accompanying drawings and having dimensions of 28 inches by 8 inches, having radiused ends and employing two layers of 0.04 inch thick Fiberfrax sheet insulating material, of which the inner layer extended downwardly about 1 inch beyond the bottom edge of the outer layer to about 1 /3 inches from the bottom edge of the mould. The metal entered the mould from a launder at a temperature of about 680690 C. and the metal level was maintained about 1 inch above the bottom edge of the sheet insulating material by means of a standard level control device. Coolant was applied at the rate of 60 gallons per minute through the mould box, which had exit slots at the bottom of the inclined surface 3 to permit the coolant water to issue and flow over the solidified surface of the ingot below the mould. With 25 commercially pure aluminium casting speeds of 3 /s3% inches per minute and with an alloy containing about 1% manganese together with the normal impurities present in commercially pure aluminium casting speeds of 3 /s3% inches per minute were achieved and exceptionally smooth-surfaced ingots were produced.

It should be understood that where thickness of insula tion material is referred to in the above description, this is the thickness of the material after compression. When referring to the thickness of Fiberfrax thermal insulation paper, this is the rated thickness as measured by the Schopper paper gauge at a compression of 8 lbs/square inch.

We claim:

1. A method of continuously casting aluminium and aluminium alloys which comprises pouring molten aluminium into an open-bottom chilled metal mould, which is initially closed by a stool, lowering the stool, applying coolant to the solidified surface of the lower portion of the ingot immediately below the lower edge of the metal mould, maintaining within the upper part of the mould during the casting operation a lining of thin flexible thermal insulation material, which is unaffected by the metal at the casting temperature, the lower edge of said lining being arranged at a distance of V81%" from the bottom edge of the mould so as to be substantially at the level at which solidification of the metal takes place at the periphery of the ingot as a result of the coolant applied directly to the surface of the ingot, and maintaining Within the mould pool of molten metal to a level above the bottom edge of the thermal insulation lining.

2. A method according to claim 1, further characterised in that said thin flexible thermal insulating material is made up of two superposed layers of thin flexible thermal insulating material, the inner layer of said two superposed layers extending below the outer layer.

3. A method according to claim 1, further characterised in that said thin flexible thermal insulation material is made up of two superposed layers of insulating material, the two layers each having a thickness of about 0.04 inch.

4. A method in accordance with claim 1 wherein said stool is lowered at a rate of about 3-4" per minute.

References Cited UNITED STATES PATENTS I. SPENCER OVERI-IOLSER, Primary Examiner.

MICHAEL V. BRINDISI, Examiner.

R. S. ANNEAR, R. A. SANDLER,

Assistant Examiners. 

1. A METHOD OF CONTINUOUSLY CASTING ALUMINIUM AND ALUMINIUM ALLOYS WHICH COMPRISES POURING MOLTEN ALUMINIUM INTO AN OPEN-BOTTOM CHILLED METAL MOULD, WHICH IS INITIALLY CLOSED BY A STOOL, LOWERING THE STOOL, APPLYING COLLANT TO THE SOLIDIFIED SURFACE OF THE LOWER PORTION OF THE INGOT IMMEDIATELY BELOW THE LOWER EDGE OF THE METAL MOULD, MAINTAINING WITHIN THE UPPER PART OF THE MOULD DURING THE CASTING OPERATING A LINING OF THIN FLEXIBLE THERMAL INSULATION MATERIAL, WHICH IS UNAFFECTED BY THE METAL AT THE CASTING TEMPERATURE, THE LOWER EDGE OF SAID LINING BEING ARRANGED AT A DISTANCE OF 7/8-1 1/4" FROM THE BOTTOM EDGE OF THE MOULD SO AS TO BE SUBSTANTIALLY AT THE LEVEL AT WHICH SOLIDIFICATION OF THE METAL TAKES PLACE AT THE PERIPHERY OF THE INGOT AS A RESULT OF THE COOLANT APPLIED DIRECTLY TO THE SURFACE OF THE INGOT, AND MAINTAINING WITHIN THE MOULD POOL OF MOLTEN METAL TO A LEVEL ABOVE THE BOTTOM EDGE OF THE THERMAL INSULATION LINING. 